Prior art reaction injection molding (RIM) process involves high pressure impingement of multi-component materials such as urethanes, epoxys, silicone, polyesters and phenolics in a liquid form which react within the mold to form solic plastic products. Pressurized flow through restricted valve orifice openings causes heating of the materials with attendant changes in viscosity, flow rates and accuracy of injection charge both as to quantity and ratio required for optimum molded product results. The general prior art system for delivering pressurized flow of multi-components to a mixing head including an improved pressure developing and balancing circuit particularly adapted for the use of chopped glass fiber reinforcement (RRIM) is disclosed in co-pending application of co-inventor James R. James, Ser. No. 06/281,401, filed on July 8, l981. The orifice system employed in such prior art mixing heads to accommodate different through-put applications involves use of multiple circular orifice openings of different size, any one of which for each material to be mixed could be moved into alignment with a matching opening in the other to cause impingement during the injection stroke.
A limitation of high pressure impingement mix heads has been found to exist, particularly in low through-put applications. In order to provide the pressures necessary for proper impingement mixing relatively small circular orifices have been necessary and as the through-put was reduced the orifice opening at some point became so small that it was subject to clogging by very small particles making a reliable production process difficult if not impossible to realize. Further it appeared that the mass of the small streams and difficulty in directing them minimized the mixing effectiveness. In addition the absence of adjustability for the individual orifices prevented monitoring of pressures and flow to maintain constant conditions in terms of volume and ratio of material for each injection stroke.
Conventional needle valve adjustment of orifice opening size is unsatisfactory due to the necessity of limiting flow in the form of a ring passage extending around the needle which facilitated clogging as compared to the equivalent passage size in the form of a open hole.